It happens here: duplicati/ReportHelper.cs at 9bb6564657686f7b7ecfe27634c2b6c138fe64e2 · duplicati/duplicati · GitHub
It doesn’t use the subjectline variable, unlike the normal flow, so it always returns the same JSON payload.
It’s a pretty tiny tweak, so I opened a PR on Github added boolean check to make the json output use the subject template … by Pectojin · Pull Request #3759 · duplicati/duplicati · GitHub
if (ExportFormat == ResultExportFormat.Json && !subjectline)